The present invention relates to a method of controlling an engine system equipped with a turbo charger, which has the function of increasing a valve overlap period during which an intake valve and an exhaust valve are both opened as a desired torque of the engine (i.e., engine load) increases, and the engine system.
It is generally known that when the valve overlap period during which the intake valve and the exhaust valve are both opened is gradually increased from a state shown in FIG. 9A to states shown in FIGS. 9B, 9C and 9D as the desired torque (load) of the engine increases (an area of valve overlap is illustrated with hatching in these figures for convenience) in the engine equipped with the turbo charger, the mount of scavenging gas so increases that the so-called “late burning (combustion)” in which the fresh air and unburned fuel are reacted in an exhaust pipe occurs and the temperature of the exhaust gas flowing to a turbine of the turbo charger increases, so that the rotational speed of the turbine increases and accordingly the rotational speed of a compressor of the turbo charger also increases. Consequently, the intake volume increases so that the engine torque can be increased.
There are three manners of increasing the valve overlap period: only advancing the opening timing of the intake valve; only retarding the closing timing of the exhaust valve; and both advancing the opening timing of the intake valve and retarding the closing timing of the exhaust valve. Herein, advancing the opening timing of the intake valve in a low engine-speed area is advantageous in increasing the intake volume. This is because advancing the opening timing of the intake valve can improve the scavenging, so that the above-described “late burning” can become properly active.
In FIG. 9, EVO is an abbreviation of Exhaust Valve Open, which means the opening timing of the exhaust valve, and EVC is an abbreviation of Exhaust Valve Close, which means the closing timing of the exhaust valve. Further, IVO is an abbreviation of Intake Valve Open, which means the opening timing of the intake valve, and IVC is an abbreviation of Intake Valve Close, which means the closing timing of the intake valve.
Herein, three experiments were conducted to inspect relationships between the valve-timing change and the torque change as shown in FIG. 10. One of them was advancing the intake-valve opening timing IVO prior to retarding the exhaust-valve closing timing EVC to increase the valve overlap period (characteristic b in FIG. 10). Another was both advancing the intake-valve opening timing IVO and retarding the exhaust-valve closing timing EVC at the same time to increase the valve overlap period (characteristic d in FIG. 10). The other was retarding the exhaust-valve closing timing EVC prior to advancing the intake-valve opening timing IVO to increase the valve overlap period (characteristic e in FIG. 10). Consequently, it was verified that a torque down shown by an arrow c occurs in the characteristic b. Herein, FIG. 10 is a characteristic chart with a lateral axis of crank angle (equivalent to the valve overlap period) and a vertical axis of torque.
The torque down shown by the arrow c may be caused by the followings. That is, as the scavenging-gas amount is increased by advancing the opening timing of the intake to increase the valve overlap period, the amount of air flowing into the exhaust port from the intake port increases and thereby the exhaust gas is cooled. Cooling the exhaust gas lowers the exhaust energy, so that the energy for rotating the turbine of the turbo charger decreases and thereby the torque down occurs. Thus, the torque down shown by the arrow c occurs despite increasing the valve overlap period in order to increase the engine torque. Accordingly, the smooth torque curve may not be obtained, so that the driver may feel uncomfortable feelings and thereby the drivability may deteriorate.
Meanwhile, Japanese Patent Laid-Open Publication No. 2007-40275 discloses a control of increasing the valve overlap period by retarding the closing timing of the exhaust valve at a rate which is greater than that of advancing the opening timing of the intake valve. Further, Japanese Patent Laid-Open Publication No. 2007-263083 discloses another control of increasing the valve overlap period by advancing the opening timing of the intake valve at a rate which is greater than that of retarding the closing timing of the exhaust valve. However, these controls were not enough to improve the drivability by obtaining the proper torque and the smooth torque curve in a wide driving area.